CA1300346C

CA1300346C - Superpurifier for nitrogen and process for purifying same

Info

Publication number: CA1300346C
Application number: CA000512229A
Authority: CA
Inventors: Marco Succi; Kiyoshi Nagai; Claudio Boffito; Fabrizio Doni
Original assignee: SAES Getters SpA; Taiyo Sanso Co Ltd
Current assignee: SAES Getters SpA; Taiyo Sanso Co Ltd
Priority date: 1985-06-28
Filing date: 1986-06-23
Publication date: 1992-05-12
Anticipated expiration: 2009-05-12
Also published as: KR930006690B1; NL192104B; FR2584062B1; KR870000237A; GB2177080A; SE463149B; IT1204420B; NL192104C; FR2584062A1; DE3621013A1; GB2177080B; IT8620963A0; BE904998A; GB8615619D0; NL8601692A; SE8602870D0; DE3621013C2; SE8602870L; JPS623006A; JPH0456763B2

Abstract

Abstract The present invention relates to a superpurifier for nitrogen and to a process for purifying a nitrogen gas.
The superpurifier of this invention provides means for contacting an impurity-containing nitrogen gas with a getter of an alloy consisting of from 15 to 30% by weight iron and from 70 to 85% by weight zirconium. The process of the present invention comprises contacting an impurity-containing gas with such a getter. The invention is particularly directed to a superpurifier and process for purifying an impurity-containing gas to a higher purity than that obtained by conventional purification processes.

Description

~300346 S~PERPURIFIER ~OR NITROGrN AND PROC2SS
FOR P~R~FYING SAME
[~rlor Art3 Nltrogen ls a useful gas enJoylng steadlly grawing demand ln many sectors o~ lndustry lncluding the flelds of electronlcs, chemicals, lron and steel making, and shlpbùildlng.
It has been a common lndustrlal process for the productlon of nitrogen to repeat compression of air by a compressor and adiabatic expansion of the compressed air untll llquid alr is obtained and then sub~ect it to fractional distillatlon under high pressure to produce liquld nitrogen of high purity. The product is fllled elther in liquid or gaseous form i~to cyl-inders and put on the marXet.
A typical inert gas, nltrogen is widely used in the afore=
mentioned fields to provide atmospheres for heat treatments of metals, for the manufacture of semiconductors and the like.
When lt is to be employed ln superfine microprocesslng such as in electronics industry, it must be further purlfieq by removlng impuritles to a higher purity immediately before use.
For large-volume conswnptlon in industrial-operatlons it is customary to vaporize liquid nitrogen and supply the resultlng gas through plpelines. Here the pt.oblem is how to meet the ~, '.

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requirement of rapid and posltlve removal of the lmpurlties, such as oxygen, hydrogen, carbon monoxlde, carbon dloxide, h~drogen, hydrocarbons, and water, from the gasified nitrogen.
In order to remove these impurltles and purify nitrogen to hi8h purity, various nitrogen gas purlfiers have heretofore been marketed and used. For instance, one of the applicants, Taiyo Sanso Co., has since 1974 sold gas purifiers (~lodels TI~-lO, -~0, -60, -100, -200, -300, -400, and -500). These and other co~mercially available gas puriflers use oxidation catalysts of metal oxides suc~. as of nic~el, chromlum, and copper to oxidize carbon mor.oxide, hydrocarbons, hydrogen and the ll~e into carbon dioxide and water and then re.~ove lmpurl-ties by adsor~lion from ~he resultants by the use of a zeolite molecular sieve, ac~ive charcoal or the like for the ~as purifi-catior.. Where high-purity nitrogen is to be simply obtained, these gas purifiers are convenient and are therefor_ in wide use.
The impurlties in tha gas purified by these existing equip-ment, according to the manufacturers' brochures, are generally as follo~s:

Constituent Oxygen Hydrocarbon Carbon dioxide Moisture ppm <O.l <O.l <0.4 <0,5 (dew point -80aC) .
' ~300346 'rO this end, the USQ`O~ hydrogen-occluding alloys, namely, T'i-r~n, Ti-~e, and rare-earth-rli alloys, have been proposed in Japanese Patenc Application Pub~lic Disclosure No. 156~08/1982.
They have, however, failed to purlfy nitrogen beyond the level tabulated above.
,Problem the Invention is to Solve]
The commercially available gas purifiers as mentioned above are simple, convenient, and efficient for obtainlng high-purity nitrogen gas. However, the recent progress of the semiconductor industry sug~ests that more ~nd more precise microprocessing and hence nitrog2n gas of even higher purity will be required for future production of highly inte2rated circuits. In fact, there is already strong dem~nd for high-purity gas for testing purposes. The technical problem the present inventlon ls lntended ~o solve is lowering the current levels of impurities according to the prior art technology to much lower levels, by one figure in parts per million.
~ r~leans for ~olving the Problem~
'~le have intensively s.udled on the rneans for purifying ni~rogen ~as to decrease its i,~lpu.ity concentrations by oneorder of m~itude in ppm each frorn the usual levels as stated above. As a result, we have arrlved at an apparatus and a process capable of purifying the conventionally purified gas of hlgh purity to an even higher purity. The present invention has now been perfected on this basls.

130034~

The apparatus according to this invention is a superpurifier for nitrogen comprising an outer shell provided with an inlet for nitrogen gas to be purified, an outlet for purified nitrogen gas, a gas flow passage connecting the gas inlet and outlet, at least one getter chamber packed with a getter of an alloy consisting of from 15 to 30% by weight of iron and from 85 to 70% by weight of zirconium and disposed midway in the gas flow passage, and neater means to maintain the getter at the temperatare at which it functions.
The process according to the invention is based on a process for purifying nitrogen characterized by the steps of first conventionally purifying impure nitrogen gas by passing it through a bed of metal oxide catalyst for oxidation at an oxidation reaction temperature and then through an adsorbent bed of zeolite molecular sieve or the like, and thereafter removing the remaining impurities by adsorption from the nitrogen gas of low impurity contents by further passing it through a getter bed packed with a getter of an alloy consisting of from lS to 30% by weight of iroh and from 35 to 70~ by weight of zirconium and maintained at a temperature of 20 to 500C.
As the getter for use in the invention which is an alloy ¢onsisting of from 15 to 30% by weight of iron and from 85 to 70%
by weight of zirconium, the one described in U.S. Patent 5pecification No. 4,306,887 may be employed.
In view of the characteristic of the getter of iron-zirco-nlum alloy that does not a~sol~bnitro~enbuta ~ rbsot~er lmpurltiesselectively, particularly deslrable ls a getter of an alloy consisting of from 22 to 25h by~weight of lron and from 75 to 78% by weight of zirconlum.
The getter o~ such an iron-zlrcon~um alloy ls substantlally a non-adsorbent for nitrogen but practically completely adsorb and remove impurities such as carbon dioxide, moisture, and hydrogen at a temperature between 20 and 500C.
The iron-zirconium composition is deslred to range from 15 to 30% by weight of iron and 85 to 70% by weight of zirco-nium. At higher percentage zirconlum contents the alloy starts to sorb significant amounts of nitrogen! the gas which is re~uired to be purified and not sorbed, whereas at lower per-centage zirconium contents the efficiency of removal (sorption) of active gases from the nitrogen is considerably reduced.
It is desirable that the getter alloy be used in the form of an intermetallic compound7 which is readily pulverized and can be handled with ease. ~loreover, the increased surface area renders the powdered material more active.
The process for the preparation of such an alloy may gene-rally conform to the procedure described in U.S. Patent Specifi-cation No. 4,312,669 that teaches the manufacture of a ternary iron-zirconium-vanadium alloy. Following practically the same procedure but omitting the addition of vanadium, a desired alloy can be made. Commercially available products, made and sold 13003~6 by SA~S Getters S.p.A. of ~llan, Italy, are approprlate for this use.
The bLnary alloy getter is packed in at least one bed zone provided midwzy ln a gas flow passage connecting an inlet for iMpure nitrogen gas and an oulet for purlfied nitrogen gas of an outer shell. The 8etter bed comblnes with a heater rneans associated with the outer shell for maintaining tha getter at its adsorption reaction temperature to constitute the essential parts of the nltrogen superpurifler accordlng to the invention.
Mi~rogen to be purified is passed through thls superpurifier so that its impure contents are brought into contact w~th the getter and are removed by adsorption.
The getter to be packed ln the chæmber ta~es ~he form o~
pelle~s in preference to fine ~articles since the former are easier to provlde sufficient interstices therebetween for the gas flow. Also, the getter as pellets of uniform size rather than small lumps irregular in size renders it easy to maintain a constant void ratio in the getter ~ed, to design the appara-tus, and to reproduce the good performance. Thus, whlle the getter in the forrn of fine particles or small lumps ls noC
obJectionaole, the use of pelle~lzed gectar, compression molded of the alloy powder, is preferred as it better meets the re-qui~ements for industrial designlng and manuracture of the nitrogen gas superpurlfier.
The heater means ~o be incorporated in the apparatus of ~300346 the invention to keep the getter hot enough for the adsorption reaction may take varied forms as will be explained later in connection with preferred embodiments of the invention. The heating method may be electric heating or indirect heating by the use of a heating medium circulated through a double-wall structure or the like. Also, the heating zone may be suitably chosen, for example, in the gas preheating region upstream of the getter bed or chamber, or around or inside the getter mass.
Since it is desirable that sufficient heating be done to effect a smooth adsorption reaction of the getter with the gas and produce as uniform a temperature distribution as feasible, the combination of the heating method and zone may be varied, according to the necessity, to best attain the end.
While it is possible that the getter chamber in the apparatus of the invention be provided inside the outer shell, as directly packed in the latter, a preferred arrangement is such that the getter bed consists of at least one cartridge packed with the getter material and which is adapted to be fitted in the outer shell detachably for ease of replacement. The getter components according to this invention adsorb and remove impurities from impure nitrogen by chemical adsorption that involves chemical changes. They therefore are consumed stoichiometrically and have a limited life. After service for a predetermined period the getter must be replaced by fresh one;
otherwise the purpose of superpurifying nitrogen will no longer ~ .

13~34~
be achleved. To this end the superpurlfier lncludlng the outer shell packed wlth the 8etter may be handled as a slngle unlt and replaced as such from time to tlme~ It is also possible to fill up the getter in a cartrldge lnstead and dlsmount the cartridge from the outer shell for replacement at proper lnter-vals of time.
The cartridge desirably employs a metal case so perforated as to facilitate the gas flow.
Because the superpurifier of the inventlon is intended to purify nitrogen until the concentrations of its ingredients as impurities zre reduced to about 0.01 ppm or less each, it is advisable that the inner wall portion of the apparatus with which nitrogen gas ccmes in contact be made of a metal polished on the surface to be close-grained and smooth enough to minimize gas adsorption and which does not form powder due to corrosion.
Such metzls include, for ex~ple, but are not limited to, stalnless steels, ~lastelloy, ~ncoloy, and l~onel metal. Any other metal material which satisfies the above requiremen~s may be su1tably chosen and used.
~ .~ sta~ed above, the inner wall material o~ the apparatus that contacts the nitrogen gas is desired to have a densely and smoothly polished surface to minimize gas adsorption. The desirable degree of s,noothness of the polished surface is numerically defined to be such that the roughness of the inner wall surface to contact nitrogen gas is 0.5 ~m or less, polyme~

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:

130034~i 0.25/lm or less in terms o~ the centerline average height tRa) ~Japanese Indu~trial Standard (JIS) B 0601-1970]. This numerical range i9 not alwayq critical but i3 recommended as a dependable, safe range.
Although the polished inner wall material is advantageously used in the zone whe-e the gas flowing out of the cartridge ch?mber comes in contact, it is, of course, possible to use it also in the zone where the $as passing throug~ the cartridge contacts. In many cases it is rather inconvenient to use the polished material only in the zone where the gas that has flowed past the cartridge contacts. The surface polishing and baking will marlfiedly shorten the time period required before h~ghly-purified gas begins to be obtained at a constant rate, even from a new apparatus.
In the appa-atus of the present invention the means for solving the technical problem before iS can be ~ariously embodied as suggested above. Thus it is to be understood that the invention is not limited to the specific embodiments thereof BO far described but various modifications may be ma~e without departing from the spirit and scope af the invention.
In the process of the invention it is essential that the nitrogen gas to be purified be passed through the bed of a metal-oxide oxidation catalyst at its oxidation reaction temperature. This is because the lack of adsorbability of ;.
g , - 13003~;

the getter used in the in~ention with re~pect to methane and other hydrocarbon~ by converting the hydrocarbons and carbon monoxide contained in the nitrogen gas into ~ater and carbon dioxide and removing most of them by adsorption by passage through 2n adsorbent bed of zeolite molecular sieve or the like.
~ he nitrogen gas puri-ied to low impurity contents by the ~nown purification process is passed through a getter bed pacXed with a getter of an alloy consisting of from 15 to 30~ by weight of iron and from 8~ to 70~ by weight of zirconiu~ and maintained at a temperature in the range of to ~00 C, so that the impurities contained in the nitrogen are ad30rbed away. If the re~ction temperature at which the impurities are removed -- 1.0 .

by adsorption from the nltrogen gas in the getter bed ls below 20C, t~e impurities are adsorbed by the getter surface but cannot be expected to dlffuse lnto the getter mass, Thus the adsorptlon practlcally comes to an end ln the state of satura-tion on the surface, wlthout fully making use of the getter capacity. In the specifled range of 20 to 500C the getter performs adsorption to the full, allowing the impuritles to diffuse thorou~hly therein. The apparent life of the getter is accordlngly extended.
On the other har.d, in the temperature region above 500C, nitrogen g~s is easily adsorbed by the getter. Setting a reac-tion temperature in excess of S00C is therefore undeslrable.
Within the specified tempe-ature r~nge o.' 20 to 500C, a narrower range of 350 lo 450C is most pre~^rred. A tempera-tu-e in the latter range is the most recommendable reaction temperature in that it assures a hi8h adsorption rate and thorougn diffusion of the impurities in~o the bed of getter wlth no possibillty of hydrogen desorption.
tEmbodlments, The present invention will no-~ be described in more detail be~ow in connection with embodiments thereof.
Nitrogen super,ourlflers embodying the lnvention are lllus-trated in Figs. 1 through 9. Fig. 1 shows a nitrogen super-purifier comprising: an outer shell 3 made of a stainless steel tube (grade SUS 304 TP conforming to Japanese Industrial Stan~-ard JIS G 3448) which has a nitrogen inlet 1 formed near the topand a nitrogen outlet 2 near the bottom, the shell being covered with a heat insulator 12 all over the surface; a top cover 14 fitted to the top of the outer shell 3; a heater 6 inserted through the top cover 14 into the space 25 inside the shell; a bed of getter 4 packed in the space defined below the heater 6 between upper and lower buffers 16, 15; and a perforated plate 7 held by a support 13 which in turn is secured to the inner wall of the outer shell and is supporting the bed as well as the perforated plate. The getter used was an iron (22-25 wt%)-zirconium (75-78%) alloy getter manùfactured and marketed by SAES
Getters S.p.A., in the form of columnar pellets having a diameter of 3 mm and height of 4 mm.
The buffers, indicated at 15, 16, consist of a layer each of small alumina spheres 4 mm in diameter packed up to a height of about 5 cm. They correct any ununiform flow af the gas through the getter bed, keep the fine particles of the getter from scattering, and uniformalize the temperature distribution.
While the embodiment being described uses small alumina spheres in forming the buffers, small stainless steel balls or a stack of fine-mesh stainless steel screens may be employed instead. Also, the buffers are not always used, and a buffer-less embodiment will be described later.
In the upper portions of the buffers 15, 16 are embedded sheathes 20, 19 accommodating thermometers 18, 17, respectively.

~300346 Chromel* and Alumel* thermocouples are used as the thermometers.
Nitrogen gas g to be purifled is introduced into the vessel at the inlet 1, heated by the heater 6, passes through the upper buffer 16 and thence, as a uniform flow, through the bed of getter 4 where it is freed from impure gas contents by adsorption. The purified gas is led through the perforated plate 7 and taken out of the vessel at the outlet 2.
Fig. 2 and following figures show other embodiments of the invention. Throughout these figures like parts are designated by like numerals and the description is omitted or minimized each.
Fig. 2 shows a superpurifier of the same construction as the embodiment in Fig. 1 excepting that an electric heater 21 is coiled round the outer shell 3 and a thermocouple 22 is installed to measure the heater temperature. This modification facilitates the temperature control of the getter bed.
Although Figs. 1 and 2 illustrate the embodiments in which the bed of getter 4 is directly packed in the outer shell 3, the getter bed may be separately provided as well. Fig. 3 shows an arrangement of cartridge 5 where the getter 4 and buffers 15, 16 ars accommodated in a cylinder equipped with perforated plates 7 at both ends. After service for a given period, the cartridge S may be taken out by removing the top cover 14 and replaced by a new one. This permits more efficient operation than with the arrangements of Figs. 1 and 2.

Trade-marks 1300~46 Fi8. 4 shcws another embodiment 11, ln whlch the outer shell 3 is of a double-wall construction, conslsting of an inner wall 24 and an outer wall 23. The space between the walls provides a passage through which a heating medium such as steam flows from a heating medlum inlet 30 to an outlet 31. In the space d-fined ~y the inner wall is accommodated a cartrid~e 5 contain-ing a getter 4, with a coil of electric heater 6 embedded ln the gette~. The heater 6 is connected to an exte,rnal power source not shown throug~ lea~s 8 (only one OL them being shown) and a terminal assembly 10. The cartridge 5 has inner and outer porous w~lls 26 concen~rically held in spaced relatlon by a support 13. The inner wall 24 of the outer shell is abutted at its lower end against a bottom pla'e with a flange 27, through which a gas inlet pipe 1 and an outlet pipe 2 extend.
The pipe 2 serves also to support the ca~tri ~é 5. Nitrogen gas 9 to be purified is fed through the inlet 1 into the outer space 25, heated there to a proper temperature, and thence forced through the porous wall 26 into the getter layer 4 for purlrlcation. The purified gas flows out into the inner space 25' and 1s taken out via the outlet 2.
Fig. S shows still another embodiment of superpurifier 11.
The oute- shell 3 is again of double-wall construction, with a space rormed therein to circulate a heating medium lntroduced at an inlet 30 and d~scharged at an outlet 31 to perform tem-perature control. Inslde the inner wall is disposed a cartridge .

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5 packed with a getter 4 between porous walls. On both sldes o~ the cartridge are arran8ed heaters 6 whlch are connected to external pcwer sources through leads 8. Impure nltrogen gas 9 is fed at an inlet 1, preheated by the heating med~um, puri~ied by passage through the getter mass 4 kept at a given temperature by the heaters 6, and then taken out at an outlet 2.
Yet another embodiment of superpurifier 11 is shown in Flg.
6. A cylindrical outer shell 3 supports a cartridge 5 by means of upper and lower plates (not shown). ~he cartridge S com-prises a built-in electric heater 6 with leads 8 and a mass of getter 4 filled in the space between upper and lower per-forated plates or bu~fer layers, with the heater embedded there-in.
Fig. 7 shows another apparatus 11 embodying the invention.
An inner cylinder is provided inside an outer shell 3 which consists of inne- and outer walls and a heat insulator 12 fllling up the sp-ace between the walls. A getter 4 is packed in ~he space bet~een the inner cyllnder and the outer shell, and an electrLc heater 6 coiled round a ceramic rod 36 is inserted into the central space in the inner cylinder. Nitrogen gas 9 to be purified enters ~he vessel at an inlet 1, passes through the getter 4, and the purified gas leaves the vessel at an outlet 2.
Fig. 8 shows another embodiment, which is a modification of the superpurifier illustrated in Fig. 3 and is characterized 1300~46 by means for recoverlng the heac of purlfled nltrogen. Nltrogen 9 ~o be purified enters a heat exchanger 28 lnstalled under the purifier body, undergoes heat exchange with the outgolng gas, and the gas so preheated moves through a plpe 29 surrounded by a heat insulator 12 and through an upper inlet 1 lnto a bed of getter 4. The purified gas is cooled in the h~at exchanger and leaves tne purifier at an outlet 2.
FLg. 9 shows a further embodiment. The outer shell ~ is a double-wall cylinder, and a heatlng medium is introduced into the space between the walls a~ an inlet 33 and is discharged at an outlet 34. Inside the outer shell 3 is disposed a gas-tight cartridge 35. The space in the cartridge case ls partl-tioned horizontally with a plurality of perforated plates 7, and a plurality of getter beds 4 are formed, each filling up the space formed by every other pair of the perforated plates.
The getter beds have electrlc heaters 6 embedded therein, one for eacn, and supplied with electricity through leads 37, 38.
Nltrogen gas 9 to be purlf~ed flows in at an inlet 1 and the purlfled gas flows out at an outlet 2.
Examples of the invention whlch used a specific getter composltlon w111 now be explalned.
The lnstrumen~s used for gas analyses in the examples were as follows:
Gas analysis instrument:
Gas chromatograph-mass spectrometer, Model TE-360B

(mf~. by Anelva Corp.) Gas chromatograph-F.I.D., Model GC-9A
(m~d. by Shimadzu Selsakusho, Ltd.) Moisture mecer:
Hygrometer, Model 700 - (mfd. by Panametric Co.) Sur~ace roughness meter:
Surfcorder, Model CE-3Y.
(mfc. by Xos~ka Laboratory Co., Ltd.) Examole l A powder-d non-evaporable getter alloy having a weight com-position o~ 70.6% zirconium and 2~.4h iron and a particle si3e of between 50 and 250 ~m were placed in the superpurifier for nitrogen shown in Fig. l. The stalnless steel (trade designa-tion, CUS 304) cylinder had an outside diameter of 21.7 mm and an inside diameter of 17.5 mm, its length belng 350 mm. The length of cyllnder occupied by the getter material was 200 mm, and the heights of the upper and lower buffers of alumina spheres were 5 cm each. Impure nltrogen gas was introduced into the superpurifier at a temperature Or 26c and a pressure of 6 kg/cm2 ~gauge) at a flow rate of 0.17 ~/min. The nitrogen flowed through the getter bed held at 375C and issued at a pressure of 4 kg/cm2 (gauge) from the outlet. Its impurlty level was measured for various gases 40 minutes after the start of the flow of the gas. The results of Table I were obtalned.

.

~ ~7 ~

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~able Im- Inlet impurity Outlet lmpurlty purity level (ppm) level (ppm) 2 0'4 0.006 CH4 O.Ol 0.01 CO 0.0~ 0.008 C2 0 04 0.007 H20 3.0 no trace The level cf impurities in the outlet g-s remained cons~ant for 1030 hours.
~xamDle 2 Pellets were produced having a diame~er of 3 mm and height of 4 mm by compressing and pelletizing a non-evaporable getter alloy having a composition and particle size identical to those of the getter alloy of Example 1. The pellets were loaded into the superpurifier sAown in Fig. 2. The stainless steel ~SUS
304) cylinder had an outer diameter of 89.1 mm and an inner diameter of 83.1 mm. Its length was 660 mm. The length of the cylinder occupied b~ the pellets of getter material, including the thicknesses of the upper and lower buffers (of alumina spheres) each having a bed height of S cm, was 18S mm.
Impure nitrogen was inlroduced into the superpurifier at a temperature of 25C and a pressure of 4 kg/cm2 (gauge) at a flow rate of 12 l/min.

, ~ 18 ~

i300~46 The lmpure nltrogen flowed through the non-evaporable 8etter bed held at a temperature of 375~C by means o~ a spiral resist-ance heater and lssued at a pressure of 3.95 kg/cm2 (gauge) , from the outlet. Its lmpurity level was measured for various gases 40 minutes after the start of the flow of nitrogen. The results obtained were as shown in Table II.
Table II

..... _ Im- Inlet impurity Outlet impurity purit-~ level (pDm) lQvel (ppm) 2 11.29 0.006 CH4 0.01 0.01 CO 8.8 0.008 C2 8.3 0.007 H20 5.0 no trace .
The level of the impurities in the outlet gas remaine8 constant for ~60 hours.
Exam~le 3 Pellets were produced exactly as ln Example 2 and place8 in the cartridge shown in Fig. 3. The cartridge had an outside dlameter of 80 mm, an inside diameter of 78 mm, and length of 244 mm. The same mass of pellets was used as in Example 2.
The cartridge was then placed in a cylinder identical to that of Example 2 (except that its iength was 719 mm). Impure nitrogen was caused to flow through the superpurifier at the same inlet pressure, temperature, and flow rate as descrlbed in Example 2. The cartrLdge was maintalned at 375~C. The outlet gas pressure and composition were found to be ldentical to those found in Ex~nple 2 at the point 40 minutes after the start of the flow of nitrogen. The level of the lmpurltles ln the outlet gas aga~n remained constant for 760 hours.
Examole 4 ~
In this e.Yo^mple the procedure of Example 2 was followed in all respects except that the inner surface roughness of the cylinder was Ra = 0 5 ~m (normally ~a = ~ m) and the stain-less st-el outlet piping had an outslde diameter 9.5 mm, inside diameter 7.5 mm, and an inner surface roughness of Ra = 0.2 ~m.
The results shown in Table III were obtained 40 minutes after the start of the flow of nitrogen.
Table III

Im- Inlet impurity Outlet impurity purity level (pom) level (pom) 2 11.29 0.003 CH4 0.01 0.01 CO 8.~ 0.008 C2 8.3 0.003 H20 5.0 no trace The level of the impurlties ln the outlet gas remained constant for 160 hours.

_ 20 -~300346 Example 5 In this example nitrogen gas to be purifled was flrst paSsed through a stainless steel (SUS 304) cyllnder having an outside diameter of 89;1 mm, inside diameter of 83.1 mm, and length of 660 mm fllled to a bed height of 185 mm wlth pelletq ~3 mm in diameter and 4 mm in length) and maintained at a temperature of 450C. Then, the water vapor content of the nitrogen gas to be purified was reduced by passing it through a dryer con-sisting of a stainless steel (SUS 301) cylinder having an out-side diameter of 89.1 mm, inside diameter of~83.1 mm, and }ength of 660 mm filled to a bed height of 200 mm with a molecular sieve typé 5-A, the pellet size being 3.2 mm across and 24 mm long. This gas was treated by the procedure of Example 2.
The outlet pressure from the dryer bed and therefore the inlet pressure to the superpurl~ier was 4 kg/cm2 (gauge); The temper-ature was varied to see the effects of dlfferent getter temper-atures. The results are given in Table IV.

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- .21 -' .

Table IV

Outlet impurity level tppm) Inlet impurity at temperature level (~pm~ 20C 250C 375C500C
2 11 . 29 0.006 0.006 0.006 0.004 CH~ 3.7 o.oos o.oo9 0.009 o.009 CO 8.8 0.008 0.008 0.008 0.004 Co2 8.3 0.007 0.007 0.007 0.004 H2O 5.0 no trace no trace no trace no trace Outlet gas remained 21 hr 1050 hr 2330 hr 2390 hr constant for Power 0 0.61 kW/h 1.1 kW/h1.7 kW/h consumption of getter cyl.

The table indicates that the getter of the invention exhibits excellent purification capability in the temperature range of 20~500C.
ples 6 and 7 Pellets were produced having a diameter of 3 mm and a length of 4 mm by compre~sion of non-evaporable getter powders conBisting of an alloy of Zr 84% and 16% by weight (Example 6~
and an alloy of Zr 71% and Fe 29% by weight (Example 7) and having particle ~izes of 50 - 250 ~m (150 ~m in average). These pellete were loaded into a superpurifier having the same . : , . .
.,.

constructlon in the samc manner as Example 2. Nltrogen ~a~
containlng impurities was 1ntroduced lnto the superpurifler at a temperature Or 25 C, pre~sure of 4 Xg/cm2 (gau~) and a flow rate of 12 }/mln.
The impurity-consisting nitrogen gas wa~ passed through the bed of the non-evaporable getter kept at a temperature of 375 C by means of a spiral re istance heater and emerged from the outlet at a pressure of 3.95 Xg/cm2 (gzuge).. The lmpurity level waq mea~ured 40 min after the start of the flow of the nitrogen gas and the results in Tab}e Y were obtained. . c Table V
.

GasInlet impurity Outlet impurity (ppm) Ex. 6 E~. 7 (ppm) (ppm) 2 11.~9 0.003 0.01 CH4 0.01 0.01 O.Ql CO 8.8 0.005 o.oog C2 8.3 0.005 0.01 ~2 5.0 no trace no trace The outlet impurity levels were con~tant for 960 hr3 and 690 hrs, respectively.

Claims

1. A superpurifier for impurity-containing nitrogen gas comprising an outer shell provided with an inlet for nitrogen gas to be purified, an outlet for purified nitrogen gas and a gas flow passage connecting the gas inlet and outlet, at least one getter chamber, a getter alloy in said chamber and consisting of from 15 to 30% by weight of iron and from 85 to 70% by weight of zirconium, being disposed in the gas flow passage, which getter chamber comprises at least one cartridge comprising a perforated metal container packed with the getter alloy, the cartridge being detachably installed in the outer shell so that it can be easily replaced by a new one, the apparatus material with which the nitrogen gas comes into contact being such that the inner wall surface to contact the gas has been polished to a surface roughness (Ra) of 0.5 µm or less in terms of the centerline average height given by the average amplitude over the entire measurement section, and means being provided for maintaining the getter alloy at its operating temperature.

2. A superpurifier as claimed in claim 1, wherein the getter alloy in the getter chamber is in the form of pellets.

3. A superpurifier as claimed in claim 1, wherein the alloy has a composition of from 22 to 25% by weight of iron and from 75 to 78% by weight of zirconium.

4. A superpurifier as claimed in any one of claims 1 to 3, wherein the alloy is an intermetallic compound of iron and zirconium.

5. A superpurifier as claimed in claim 1, including a pretreatment unit for removing hydrocarbons.

6. A superpurifier as claimed in claim 5, wherein the pretreatment unit comprises an oxidizer provided with a bed of a metal oxide catalyst for oxidizing impure nitrogen gas and an absorber provided with an adsorbent bed for adsorbing at least one of carbon monoxide, carbon dioxide, and oxidized impurities in said gas.

7. A superpurifier as claimed in claim 6, wherein the adsorbent bed comprises a zeolite molecular sieve.

8. A superpurifier for purifying an impurity-containing nitrogen gas which superpurifier comprises:

a. an outer shell having a gas inlet through which the impurity-containing nitrogen gas enters the superpurifier and a gas outlet through which a purified nitrogen gas leaves the superpurifier;

b. a gas flow passage within the outer shell extending from the gas inlet to the gas outlet thereby providing fluid communication between the gas inlet and the gas outlet;
c. a getter chamber disposed in the gas flow passage between the gas inlet and the gas outlet, which getter chamber comprises at least one cartridge comprising a perforated metal container;
d. a getter material provided in the getter chamber, the getter material being an alloy of from 15 to 30% by weight iron and 85 to 70% by weight zirconium; and e. means for heating the getter material and maintaining the getter material at a temperature at which the getter material selectively sorbs impurities from the impurity-containing nitrogen gas without sorbing nitrogen;
f. the apparatus material with which the nitrogen gas comes into contact being such that the inner wall surface to contact the gas has been polished to a surface roughness (Ra) of 0.5 µm or less in terms of the centerline average height given by the average amplitude over the entire measurement section.

9. A superpurifier for purifying an impurity-containing nitrogen gas, which superpurifier comprises:
a. an outer shell having a gas inlet and a gas outlet;

b. a gas flow passage within the outer shell, said gas flow passage extending from the gas inlet to the gas outlet thereby providing fluid communication between the gas inlet and the gas outlet;
c. a cartridge detachably mounted in the gas flow passage in the outer shell, said cartridge comprising a perforated metal container packed with a getter material, the getter material comprising an alloy of from 22 to 25% by weight iron and from 75 to 78% by weight zirconium and the getter material being in the form of columnar pellets having a diameter of approximately 3 mm and a height of 4 mm; and d. means for maintaining the getter material at a temperature of from 350° to 450°C;
e. the apparatus material with which the nitrogen gas comes into contact being such that the inner wall surface to contact the gas has been polished to a surface roughness (Ra) of 0.5 µm or less in terms of the centerline average height given by the average amplitude over the entire measurement section.